Improving mechanical properties and reducing internal stresses (induced by polymerization shrinkage) have been among the major research efforts for polymeric dental restorative composites for decades. The recently fast developing technology of "electrospinning" and its novel product "polymer nanofibers", provide a potential solution to solve these problems. Polymer nanofibers, with diameters ranging from several nanometers to several hundred nanometers, can be made from a variety of polymer solutions or melts. In the process of electrospinning, the key phenomenon "bending instability" results in extremely large elongational flow rate of up to 1,000,000 s(-1). Such huge elongational flow rate can well align macromolecule chains as well as nanofillers (e.g., layered silicates) along the nanofiber axis; therefore, electrospun nanofibers can be extraordinary strong. The electrospun polymer nanofibers, typically collected as non-woven fabrics, can be soaked with and embedded into dental monomers (e.g., Bis-GMA/TEGDMA mixture). After polymerization, the composite resins are in the form similar to the interpenetration network (IPN). In this research, for the first time, the electrospun polymer nanofibers will be introduced to prepare dental materials. In particular, the use of strong nanofibers of Nylon 6 and/or Nylon 6/layered silicate nanocomposites (NLS) to improve the strength, and the use of elastomeric nanofibers of ethylene-propylene-diene elastomer (EPDM) to reduce the internal stresses, will be studied. Other objectives include (1) investigating the process of electrospinning to make nanofibers with desirable morphological and physical properties, (2) improving interfacial properties between the filler of nanofibers and the matrix resin, through both chemical modifications of the macromolecules and plasma surface treatment of the nanofibers, (3) evaluating hybrid embedding of Nylon 6, NLS and EPDM nanofibers to maximize strength, and to minimize internal stress, and (4) characterizing long-term mechanical and physical properties, such as wear, water aging and thermal cycling, for optimal composite resins with desirable properties (e.g. high strength and/or minimum internal stresses). This project will provide (1) the mechanisms of electrospun polymer nanofiber reinforcement and reduction of internal stress for dental resins; (2) guidelines on microstructural design and processing of nanofiber reinforced and/or toughened dental composite resins; and (3) a basis for the next generation of high performance composite resins for dental restorative (and potentially other biomedical) applications.